Table of Contents
Pressure
Pressure is a measurement of the ratio between the force applied to the surface area on which that force is applied. The SI pressure units are Pascals which are equivalent to Newtons / Meters2. Among many other units of pressure, some of the most common used within engineering include bar, atm, torr, psi, and mmHg. There are many different types of pressure as essentially anything can exert a force on a surface although within the context of chemical engineering fluid pressure is the most common which can be the pressure of either a liquid or vapour.
Fluid pressure within a container can be defined by the equation where Patm is the atmospheric pressure exerted at the top of the container, is the density of the fluid, g is the force of gravity, and h is the height of the fluid within the container. Atmospheric pressure is evidently the pressure within the atmosphere of the earth and has been measured to be 101325 Pa / 760 mmHg / 1.01325 bar / 14.696 psi at sea level. As shown in figure 1 atmospheric pressure is applied at the top of the container onto the fluid. The density of the fluid is another large factor as the higher the density of the fluid the higher the pressure will be. For example water is more dense than gasoline and will exert a lot more pressure. For a chemical engineer, gravity will be constant at 9.8 m/s2 although perhaps in the future if there is a demand for water tanks on mars or the moon, this value will change. Height is another large factor in determining pressure and can often change. In figure 1, the height is measured from the top of the water down which means the pressure will be the greatest at level C and smallest at level A.
Liquid pressure and vapor pressure are both very different in the ways they apply force to a container. As shown in Figure 1, liquid pressure is a combination of atmospheric pressure and the amount of a fluid in a container applying pressure to the bottom. The height determines the liquid pressure within a container whether they container is as thin as a straw or as wide as a pot, if the height of both containers is 15 cm, the pressure at a point at the bottom of each container will be the same. Vapour pressure is different as the molecules within a gas bumping into the walls of the closed container is the pressure being measured as shown in Figure #.
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Temperature
Temperature can be defined as the average kinetic energy of all the molecules in a substance. The kinetic energy of molecules cannot be measured directly therefore temperature is determined by measuring other physical properties which are affected by temperature. There are many different devices that use different ways to measure temperature because of this. A resistance thermometer measures the electrical resistance of a conductor, a thermocouple measures the voltage at the connection point of 2 different metals, a pyrometer measures the radiation of the substance, and the most common type of tool a thermometer measures change in volume of a fluid. Among many different tools of measuring temperature, there are also many different temperature scales. The Celsius and Fahrenheit scales are based off setting values to the freezing and boiling points of water while the Kelvin and Rankine scales are do the same thing although their absolute zero temperature is at zero unlike the Celsius and Fahrenheit scales. The conversions between temperatures can be found in Figure # and the key temperatures can be found in table 1.
| Table 1 | Melting Point | Boiling Point | Absolute Zero |
|---|---|---|---|
| Celsius (°C) | 0 | 100 | -273.15 |
| Fahrenheit (°F) | 32 | 212 | -459.67 |
| Kelvin (K) | 273.15 | 373.15 | 0 |
| Rankine (°R) | 491.67 | 671.67 | 0 |
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Level
-high high, high, low level alarms, level in tanks,
Flow
Flow rate is an essential measurement within many chemical processes and can be used in many different ways. It is usually depicted using a point over a variable for example mass flow rate can be represented as such in Figure 3. Flow rate can be defined as the rate at which a substance travels from one point to another point and can be measured either using volume (volume/time) or mass (mass/time). Mass and volume are not independent measurements as one can be calculating from the other using the density of whichever fluid is being measured. The relationship between mass and volume can be found in Figure 4. There are many different tools that can be used to measure the flow rate of a substance through a pipe although rotameters and orifice meters are the most common. A rotameter measures flow rate by determining the height of an object when the substance is flowing upward as depicted in Figure 5. An orifice meter measures the pressure drop of a substance before an after travelling through a small opening as shown in Figure 6. The large pressure drop indicates a large flow rate while a small pressure drop indicates a small flow rate.
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| Figure 4 |
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| Figure 5 |
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| Figure 6 |
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Mass
Mass is simply the measurement of how much matter a substance or object contains. It is often confused with the term weight although there is a distinct difference between the two terms. When someone stands on a scale, the scale is feeling the force from that person along with the force of gravity which means said scale is measuring weight which could be in kg/(m/s2) or similar units. Your mass is then calculated by the scale and displayed to you in kilograms or another unit of mass. Since weight is a measurement of the force of gravity on an object, such as a human in the example given, your weight would not be the same on a different planet although your mass would be. Common units of mass include kilograms (kg), grams (g), pound mass (lbm), ounces (oz), and tons (t). Density is another measurement related to mass as it is the ratio of mass to a unit of volume such as g/cm3, kg/m3, and lbm/ft3.
Composition
Dissolved Oxygen
The amount of dissolved oxygen within a substance is another measurement often used within chemical processes. The three most common techniques of measuring dissolved oxygen are modern-day electrical or optical sensors, the colorimetric method, and Winkler titration.
pH
Salinity
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